Seat belt should not be through there.
The post on the seat will lock it into the bracket when you push the lever back.
If you do a search on the forums or the site, someone posted a PDF with the measurements at some point in time.
Can you shoot a picture of your front brackets? You look off enough that the front brackets may be mounted backwards.
Can't believe how many people mount these seats wrong...mine was as well and had to have my local shop fix it...
Given they are a weird design...but come one...google an install manual already!!
1994 D-90 ST #734
1997 Disco I
1988 Range Rover 40th Anniversary
1999 Disco II
1995 D-90 ST #2731
1997 Range Rover 4.6 HSE Vitesse
2003 Range Rover HSE